- Email: [email protected]
Engineering interfaces in cuprate superconductors
ARTICLE IN PRESS
Physica B 403 (2008) 1149–1150 www.elsevier.com/locate/physb
Engineering interfaces in cuprate superconductors Gennady Logvenova, Vladimir V. Butkoa, Catherine DevilleCavellinb, Jiwon Seoc, Adrian Gozara, Ivan Bozovica, a Brookhaven National Laboratory, Upton, NY 11973, USA Faculte´ des Sciences et Technologies, Universite´ Paris 12-Val de Marne, 94010 CRETEIL-Cedex, France c Department of Physics, Cavendish Laboratory, Cambridge CB3 0HE, UK
b
Abstract Using an advanced molecular beam epitaxy system for atomic-layer engineering of complex oxides, we have fabricated a variety of superlattices with stacked layers of La2xSrxCuO4 doped to different levels. In superlattices formed by stacking highly overdoped, metallic La1.5Sr0.5CuO4 and insulating La2CuO4 layers we have observed superconductivity at temperature as high as 30 K, even though neither of the building blocks was superconducting. Different possible mechanisms of this superconductivity are discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: High-temperature superconductivity; Molecular beam epitaxy; Artificial superlattices
1. Introduction High-temperature superconductors (HTS) are intrinsically multi-layered materials where metallic CuO2 layers are stacked with other metal-oxide layers, so called ‘‘charge-reservoir blocks’’, that provide mobile holes to CuO2 layers. It is tempting, although challenging experimentally, to try creating new HTS compounds by growing artificially multi-layered structures with different stacking layers. The technique we use is atomic layer-by-layer molecular beam epitaxy (ALL-MBE) [1]. It provides for precise stoichiometry and thickness control and enables material engineering at various levels—down to a single atomic layer. In this study we report on epitaxial growth of atomically smooth La2xSrxCuO4 (LSCO) films over a broad range of doping levels (x ¼ 0–0.50), as well as a variety of artificial superlattices. The later include some that were built by alternating layers of highly overdoped, metallic but not superconducting La1.5Sr0.5CuO4 and undoped, insulating La2CuO4 (LCO). In what follows, we refer to these as [n LSCO:m LCO]k superlattices, where n and m are the numbers of unit cells of LSCO and LCO layers, Corresponding author. Tel.: +1 631 344 4973; fax: +1 631 344 4071.
E-mail address: [email protected] (I. Bozovic). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.351
respectively, in one superlattice period, and k is the number of superlattice periods. We have observed superconductivity with the critical temperature TcE30 K even in the finest, [1 LSCO:1 LCO]k superlattices.
2. Experimental Our ALL-MBE system has been described elsewhere [2]. We have studied hundreds of single-phase LSCO films doped to different levels. Reproducibility of the film quality and transport properties is excellent. Atomically smooth films without secondary phase precipitates are grown with yield close to 100%. The r.m.s. surface roughness as low as 2–3 A˚ is observed by atomic-force microscopy [3]. We have grown [n LSCO:m LCO]k superlattices on LaSrAlO4 substrates at 700 1C and 8 106 Torr ozone pressure. The typical reflection high energy electron diffraction (RHEED) intensity oscillations are shown in Fig. 1 for six periods of the [1 LSCO:1 LCO] superlattice. After growth, the films were annealed in high vacuum at 200 1C for half an hour to remove excess oxygen from the LCO layers. The resonant X-ray scattering (P. Abbamonte and S. Smadici, unpublished data) shows that the interfaces between LSCO and LCO layers are sharp—there is no significant Sr diffusion from one layer to the other.
ARTICLE IN PRESS
LSCO
LCO
200 0
1.0
106 104 102
0
50 100 150 200
0
50 100 150 200
100
0.9 40 0.8
LSCO-LCO superlattice 30
0.7
R(Ohm)
Normalized RHEED Intensity
1.1
400
ρ(10−3Ohm cm)
ρ(10−6Ohm cm)
LCO
LSCO
LCO
LCO
LSCO
LCO
LSCO
LCO
LSCO
LCO
LSCO
LSCO
G. Logvenov et al. / Physica B 403 (2008) 1149–1150
1150
LSCO: La1.5Sr0.5CuO4
0.6 0.5
20 10
0
500
1000
1500
Time (s) Fig. 1. RHEED intensity oscillations during growth of six periods of [1 LSCO:1 LCO]k superlattice.
The resistance of single-phase LSCO and LCO films and [n LSCO:m LCO]k superlattices was measured using the standard four-point contact method. The typical temperature dependence of resistivity for (a) a highly overdoped LSCO films and (b) insulating undoped LCO film, are shown in Fig. 2; neither shows any sign of superconductivity down to 4 K. However, as it is seen in Fig. 2(c), the [1 LSCO:1 LCO]k superlattice is superconducting with TcE30 K. The most interesting open question here, and the subject of our ongoing research, is whether this superconducting transition is due to charge reconstruction (depletion and accumulation) at the interface due to the difference in electrochemical potential and band bending, to oxygen doping at the interface (perhaps enabled by these electrostatic interactions) or to Sr diffusion from overdoped
0 0
50
100 T(K)
150
200
Fig. 2. Resistivity versus temperature for (a) overdoped LSCO film (x ¼ 0.5), (b) undoped LCO films, and (c) [1 LSCO:1 LCO]20 superlattice.
layer to underdoped and formation of very thin nearlyoptimally-doped layer. Acknowledgment This work was supported by DOE (Contract MA-509MACA). References [1] I. Bozovic, J. Eckstein, G. Virshup, Physica C 235 (1994) 178. [2] I. Bozovic, IEEE Trans. Appl. Supercond. 11 (2001) 2686. [3] I. Bozovic, G. Logvenov, et al., Phys. Rev. Lett. 89 (2002) 107001; I. Bozovic, G. Logvenov, et al., Nature 422 (2003) 873; I. Bozovic, G. Logvenov, et al., Phys. Rev. Lett. 93 (2004) 157002.
Physica B 403 (2008) 1149–1150 www.elsevier.com/locate/physb
Engineering interfaces in cuprate superconductors Gennady Logvenova, Vladimir V. Butkoa, Catherine DevilleCavellinb, Jiwon Seoc, Adrian Gozara, Ivan Bozovica, a Brookhaven National Laboratory, Upton, NY 11973, USA Faculte´ des Sciences et Technologies, Universite´ Paris 12-Val de Marne, 94010 CRETEIL-Cedex, France c Department of Physics, Cavendish Laboratory, Cambridge CB3 0HE, UK
b
Abstract Using an advanced molecular beam epitaxy system for atomic-layer engineering of complex oxides, we have fabricated a variety of superlattices with stacked layers of La2xSrxCuO4 doped to different levels. In superlattices formed by stacking highly overdoped, metallic La1.5Sr0.5CuO4 and insulating La2CuO4 layers we have observed superconductivity at temperature as high as 30 K, even though neither of the building blocks was superconducting. Different possible mechanisms of this superconductivity are discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: High-temperature superconductivity; Molecular beam epitaxy; Artificial superlattices
1. Introduction High-temperature superconductors (HTS) are intrinsically multi-layered materials where metallic CuO2 layers are stacked with other metal-oxide layers, so called ‘‘charge-reservoir blocks’’, that provide mobile holes to CuO2 layers. It is tempting, although challenging experimentally, to try creating new HTS compounds by growing artificially multi-layered structures with different stacking layers. The technique we use is atomic layer-by-layer molecular beam epitaxy (ALL-MBE) [1]. It provides for precise stoichiometry and thickness control and enables material engineering at various levels—down to a single atomic layer. In this study we report on epitaxial growth of atomically smooth La2xSrxCuO4 (LSCO) films over a broad range of doping levels (x ¼ 0–0.50), as well as a variety of artificial superlattices. The later include some that were built by alternating layers of highly overdoped, metallic but not superconducting La1.5Sr0.5CuO4 and undoped, insulating La2CuO4 (LCO). In what follows, we refer to these as [n LSCO:m LCO]k superlattices, where n and m are the numbers of unit cells of LSCO and LCO layers, Corresponding author. Tel.: +1 631 344 4973; fax: +1 631 344 4071.
E-mail address: [email protected] (I. Bozovic). 0921-4526/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2007.10.351
respectively, in one superlattice period, and k is the number of superlattice periods. We have observed superconductivity with the critical temperature TcE30 K even in the finest, [1 LSCO:1 LCO]k superlattices.
2. Experimental Our ALL-MBE system has been described elsewhere [2]. We have studied hundreds of single-phase LSCO films doped to different levels. Reproducibility of the film quality and transport properties is excellent. Atomically smooth films without secondary phase precipitates are grown with yield close to 100%. The r.m.s. surface roughness as low as 2–3 A˚ is observed by atomic-force microscopy [3]. We have grown [n LSCO:m LCO]k superlattices on LaSrAlO4 substrates at 700 1C and 8 106 Torr ozone pressure. The typical reflection high energy electron diffraction (RHEED) intensity oscillations are shown in Fig. 1 for six periods of the [1 LSCO:1 LCO] superlattice. After growth, the films were annealed in high vacuum at 200 1C for half an hour to remove excess oxygen from the LCO layers. The resonant X-ray scattering (P. Abbamonte and S. Smadici, unpublished data) shows that the interfaces between LSCO and LCO layers are sharp—there is no significant Sr diffusion from one layer to the other.
ARTICLE IN PRESS
LSCO
LCO
200 0
1.0
106 104 102
0
50 100 150 200
0
50 100 150 200
100
0.9 40 0.8
LSCO-LCO superlattice 30
0.7
R(Ohm)
Normalized RHEED Intensity
1.1
400
ρ(10−3Ohm cm)
ρ(10−6Ohm cm)
LCO
LSCO
LCO
LCO
LSCO
LCO
LSCO
LCO
LSCO
LCO
LSCO
LSCO
G. Logvenov et al. / Physica B 403 (2008) 1149–1150
1150
LSCO: La1.5Sr0.5CuO4
0.6 0.5
20 10
0
500
1000
1500
Time (s) Fig. 1. RHEED intensity oscillations during growth of six periods of [1 LSCO:1 LCO]k superlattice.
The resistance of single-phase LSCO and LCO films and [n LSCO:m LCO]k superlattices was measured using the standard four-point contact method. The typical temperature dependence of resistivity for (a) a highly overdoped LSCO films and (b) insulating undoped LCO film, are shown in Fig. 2; neither shows any sign of superconductivity down to 4 K. However, as it is seen in Fig. 2(c), the [1 LSCO:1 LCO]k superlattice is superconducting with TcE30 K. The most interesting open question here, and the subject of our ongoing research, is whether this superconducting transition is due to charge reconstruction (depletion and accumulation) at the interface due to the difference in electrochemical potential and band bending, to oxygen doping at the interface (perhaps enabled by these electrostatic interactions) or to Sr diffusion from overdoped
0 0
50
100 T(K)
150
200
Fig. 2. Resistivity versus temperature for (a) overdoped LSCO film (x ¼ 0.5), (b) undoped LCO films, and (c) [1 LSCO:1 LCO]20 superlattice.
layer to underdoped and formation of very thin nearlyoptimally-doped layer. Acknowledgment This work was supported by DOE (Contract MA-509MACA). References [1] I. Bozovic, J. Eckstein, G. Virshup, Physica C 235 (1994) 178. [2] I. Bozovic, IEEE Trans. Appl. Supercond. 11 (2001) 2686. [3] I. Bozovic, G. Logvenov, et al., Phys. Rev. Lett. 89 (2002) 107001; I. Bozovic, G. Logvenov, et al., Nature 422 (2003) 873; I. Bozovic, G. Logvenov, et al., Phys. Rev. Lett. 93 (2004) 157002.